ASTM D7757 Standard Test Method for Silicon in Gasoline and Related Products by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry
3. Summary of Test Method
3.1 A monochromatic X-ray beam with a wavelength suitable to excite the K-shell electrons of silicon is focused onto a test specimen contained in a sample cell (see Fig. 1). The fluorescent Kα radiation at 0.713 nm (7.13 Å) emitted by silicon is collected by a fixed monochromator (analyzer). The intensity (counts per second) of the silicon X-rays is measured using a suitable detector and converted to the concentration of silicon (mg/kg) in a test specimen using a calibration equation.
4. Significance and Use
4.1 This test method provides rapid and precise measurement of total silicon in naphthas, gasoline, gasoline-ethanol blends, RFG, ethanol and ethanol-fuel blends, and toluene with minimum sample preparation. Typical analysis time is 5 min to 10 min per sample.
4.2 Excitation by monochromatic X-rays reduces background, simplifies matrix correction, and increases the signal/background ratio compared to polychromatic excitation used in conventional WDXRF techniques.
4.3 Silicone oil defoamer can be added to coker feedstocks to minimize foaming in the coker. Residual silicon in the coker naphtha can adversely affect downstream catalytic processing of the naphtha. This test method provides a means to determine the silicon content of the naphtha.
4.4 Silicon contamination of gasoline, gasoline-ethanol blends, denatured ethanol, and their blends has led to fouled vehicle components (for example, spark plugs, exhaust oxygen sensors, catalytic converters) requiring parts replacement and repairs. Finished gasoline, gasoline-ethanol blends, and ethanol-fuel blends can come into contact with silicon a number of ways. Waste hydrocarbon solvents such as toluene can be added to gasoline. Such solvents can contain soluble silicon compounds. Silicon-based antifoam agents can be used in ethanol plants, which then pass silicon on to the finished ethanol-fuel blend. This test method can be used to determine if gasoline, gasoline-ethanol blends, and ethanol-fuel blends meet specifications with respect to silicon content of the fuel, and for resolution of customer problems.
4.5 Some silicon compounds covered by this test method are significantly more volatile than the silicon compounds typically used for the preparation of the calibration standards. Volatile compounds may not meet the stated precision from this test method because of selective loss of light materials during the analysis.
5. Interferences
5.1 Differences between the elemental composition of test samples and the calibration standards can result in biased silicon determinations. For fuels within the scope of this test method, the only important elements contributing to bias resulting from differences in the matrices of calibrants and test samples are hydrogen, carbon, and oxygen. A matrix-correction factor (C) may be used to correct this bias; the calculation is described in Annex A1. For general analytical purposes, the matrices of test samples and the calibrants are considered to be matched when the calculated correction factor C is within 0.95 to 1.05. No matrix correction is required within this range. A matrix correction is required when the value of C is outside the range of 0.95 to 1.05. For most testing, matrix correction can be avoided with a proper choice of calibrants. For example, Fig. 2 and the calculation in Annex A1 show that a calibrant with 87.5 % by mass carbon and 12.5 % by mass hydrogen can cover non-oxygen containing samples with C/H ratios from 5.0 to 11.0, which corresponds to a correction factor range of 0.95 to 1.05.
5.2 Fuels containing large quantities of ethanol, such as ethanol fuel blends, denatured fuel ethanol, and gasoline-ethanol blends (see Specifications D4806 and D5798), can have a high oxygen content leading to significant absorption of silicon Kα radiation and low silicon results. Such fuels may be analyzed using this test method provided either that correction factors (see Table 1 and Table 2) are applied to the results or by using calibration standards that are matrix matched to the test sample. For gasoline samples with oxygenates, up to 3.1 % by mass oxygen can be tolerated for test samples with the same C/H ratio as the calibrants.
5.2.1 For test samples with high oxygenate content, such as denatured fuel ethanol and ethanol fuel blends (see Specifications D4806 and D5798), ethanol-based calibrants may be used provided the correction factors as described in 5.1 are applied to the results. Table 1 and Table 2 show the correction factor that should be applied to the measurement results of the gasoline-ethanol and ethanol fuel blends if they are measured using either an isooctane or ethanol calibration curve.
NOTE 3 - Alcohol based calibration standards may be preferred for test samples containing a high oxygenate content.
5.3 To minimize any bias in the results, use calibration standards prepared from silicon-free base materials of the same or similar elemental composition as the test samples.
5.3.1 When diluting samples, use a diluent with an elemental composition the same or similar to the base material used for preparing the calibration standards.
5.3.2 A base material for gasoline may be simulated by mixing 2,2,4-trimethylpentane (isooctane) and toluene in a ratio that approximates the expected aromatic content of the samples to be analyzed.
